A marine transportation using ships as a goods movement means to a remote area consumes less energy compared with other transportation and incurs a low transportation cost, so it takes a large portion of global trade.
Recently, a marine transportation such as a container carrier uses a large ship in order to improve the efficiency of transportation, and the use of the large ship increases the volume of traffic of ships to secure economic efficiency of transportation. Thus, more harbors having mooring facilities for allowing a large ship to come alongside the pier and loading and unloading facilities are increasingly required.
However, harbors allowing a large container ship to come alongside the pier are limited around the world, and construction of such a harbor incurs much cost due to dredging or the like for maintaining the depth of water in the harbor and requires a spacious area. In addition, the construction of a big harbor causes traffic congestion nearby or greatly affects the surrounding environment such as damage to a coastal environment, leaving a variety of restrictions to the construction of a big harbor.
Thus, research into a mobile harbor allowing a large ship to anchor in the sea away from the land and ship and load cargo, rather than making a large ship to come alongside the pier in the harbor, is under way.
FIG. 1 is a schematic view showing that a container C handling operation with respect so a container carrier S is performed by a crane 1 installed in a ship 50 serving as a mobile harbor. Here, a widthwise direction of a boom 10 (or a lengthwise direction of the ship 50) is defined as a lateral direction (X direction in the figure), and a lengthwise direction of the boom 10 (or a widthwise direction of the ship 50) is defined as a longitudinal direction (Y direction in the figure).
In general, the crane 10 comprises a spreader 30 gripping a container C and moved in the vertical direction, a trolley 20 supporting the spreader 30 and moved in the longitudinal direction and the boom 10 guiding the trolley 20 to enable she trolley to be moved.
FIG. 2 is a view showing schematically a method for tying a hoist wire of a conventional crane. A spreader is moved in the vertical direction by using a hoist wire system provided on a boom B. The hoist wire system includes a wire drum B1 winding/unwinding a hoist wire W and a various kinds of sheave blocks B2, B3, T1 and S1 for changing a direction of the hoist wire W. A spreader is coupled to the sheave block S1. The hoist wire W is tied such that a vertical level of the spreader is not changed even though a trolley 1 is moved.
In the meantime, a pitching and a roiling of the ship on the sea are indispensably occurred due to a wind, wave or tidal current, and the like. Referring to FIG. 1, in the conventional crane 1, the trolley 20 moved on the crane boom 10 and the spreader 30 mounted to the trolley can be moved only the longitudinal direction. Accordingly, when a relative location between the crane 1 or the ship 50 and the container ship S is not maintained due to a pitching and rolling of the ship, it is difficult to couple or decouple the spreader 30 with or from the container. To correct the above defect, the crane 1 itself or the ship 50 itself should be moved, so that it is not easy to control the crane or the ship and power is excessively consumed.